In the manufacture of semiconductor electroluminescent devices, such as lasers and light emitting diodes, it is known to apply one or more layers of light reflective material to at least one of the radiation emitting facets of the device. These facet coatings, which can be of Al.sub.2 O.sub.3, Si, SiO.sub.2, Si.sub.3 N.sub.4, MgF.sub.2 and the like, can enhance the radiant energy emitted by the device and decrease the current density required for lasing to occur by increasing, or varying in a controlled manner, the reflectivity of the facets. Facet coatings can also increase the lifetime of a laser device by providing protection against catastrophic facet degradation and/or facet erosion.
During the fabrication of laser devices a processed semiconductor wafer is typically cleaved into bars whose width corresponds to the desired width of the devices. The appropriate facet coatings are applied to the bars for ease of handling before dividing into devices. The top and bottom surfaces of the bar should be masked because an "overspray" of the facet coatings onto these surfaces adversely affects the mechanical, thermal and electrical properties of the subsequent solder joint or bond which holds each device to its heatsink mount. The bars can be mounted in fixtures designed to mask the top and bottom surfaces of the bar, exposing only the facets to the facet coating process which is carried out by any convenient means, such as electron beam evaporation. Typically the top surface of the bar is butted against a flat surface on the inside of the fixture so that no deposition of the facet coatings can occur on this top surface. A slot or window in the fixture exposes only as much of the facet of the bar as is necessary. The ability of the fixture to prevent an overspray of the facet coating onto the top surface relies heavily on an intimate contact of the top surface with the inside of the fixture. Thus the top surface of the bar must be planar for proper masking.
However, because the structural layers of the laser are deposited by liquid phase epitaxy (LPE), or the like, the "grown" surface is usually somewhat non-planar. Further, it is well known to process the device so as to include an oxide defined stripe contact which introduces additional non-planarities. Finally, many recent devices are fabricated by growing layers over intentionally non-planar substrates. Thus, fixtures used to mask a grown surface during facet coating cannot be made to conform to the non-planarities, and overspray of the particular facet coatings onto the grown surface still occurs.
Various mask coatings including waxes, photoresists and varnishes have been applied to the grown surface of a processed wafer prior to cleaving it into bars. Although the overspray is prevented from depositing directly onto the grown surface, it does deposit over the masking coating. In many cases, upon removing the masking material with solvents, the overspray still remains. This is especially true when a plurality of facet coatings have been deposited leaving a thick overspray. Attempts to physically remove this hinge of overspray usually results in tearing away part of the facet coating from the facet. Chemical etches are obviously not a solution because whatever attacks the overspray, will also attack the facet coating itself.
It would be desirable therefore to have a method of selectively applying a coating to a body of semiconductor material which alleviates the problem of overspray onto surfaces which are to be kept free of the coating.